1. Field of the Invention
This invention relates to a colorless, transparent, high purity and good crystallinity synthetic diamond substantially free from impurities, crystal defects, strains, etc., a process for the production of the same and a method of measuring the strain of diamond.
2. Description of the Prior Art
Diamond crystals can be applied to various uses such as heat sinks, dies for wire drawing, cutting tools for precision working, optical parts, laser windows and anvils for producing ultra-high pressures, because of having high hardness, high strength, excellent thermal conductivity, excellent corrosion resistance and good transmittance of lights.
Naturally occurring diamonds, most of which are called Ia type, contain about 1000 ppm of nitrogen. The nitrogen in this natural diamond is distributed in the crystal in an aggregated form, so that crystal defects and internal strains are large and there occurs absorption of light due to nitrogen in the infrared range. Depending upon the variety of a rough stone, there is a large dispersion. Thus, the applied use has been limited to heat sinks or tools. High purity natural diamond containing nitrogen impurity in an amount of at most several ppm is called IIa type and such diamond is so little as represented by, for example, an output of about 2% based on the whole rough stones. Since the natural diamond of IIa type contains very small amount of impurities, is colorless and transparent and has superior transmittance property, it has widely been applied to jewels, optical parts and laser window materials.
However, there remain defects or strains to a considerably great extent in the interior part of the natural diamond because of undergoing complicated growth progresses in the interior part of the earth. As to the strain, the natural diamond rather has it more than synthetic diamonds containing nitrogen. Furthermore, the natural diamond of IIa type has such a problem that the output quantity is small to result in a higher production cost and difficulty in obtaining.
An ordinary diamond artificially synthesized under an ultra-high pressure and high temperature is called type Ib and contains several hundreds ppm of nitrogen. Since this nitrogen is contained in the diamond crystal as an isolated substitutional impurity, the crystal is rendered yellow and less valuable for jewels. In addition, the concentration of nitrogen is largely different depending on the growth sectors and the nitrogen distribution is largely uneven in the interior part of the crystal, thus resulting in more strains in the crystal.
On the other hand, it is known that when a nitrogen getter such as Al is added to a solvent metal during synthesis of diamond, the nitrogen in the diamond can be removed to about several ppm to obtain diamond of IIa type. When the nitrogen getter is added to the solvent metal, however, inclusions in large amounts ordinarily tend to be taken in the crystal to largely decrease the production yield of a good quality crystal. Accordingly, the production cost of the synthetic diamond of IIa type is higher than that of the natural diamond of IIa type. Removal of nitrogen in the synthetic diamond is limited to about 1 ppm and the estimation of the diamond as a decorative article is approximately H to J by GIA scale (Japanese Patent Laid-Open Publication No. 88289/1977). Moreover, there is found an absorption due to nitrogen in the ultraviolet range.
As illustrated above, in the synthetic diamond of the prior art, a crystal hardly containing nitrogen and substantially free from inclusions or internal defects has not been known.
Furthermore, it is well known that an element such as Ti or Zr is used as a nitrogen getter. When using such an element as a nitrogen getter, nitrogen can effectively be removed, but carbides such as TiC, ZrC, etc. are formed in large amounts in a solvent and taken in a diamond crystal, so that a good quality diamond can hardly be obtained.
On the contrary, the inventors have succeeded in producing an inclusion-free synthetic diamond of IIa type having a nitrogen content of at most 0.1 ppm by using at least one element selected from Group IVa and Va elements having a high nitrogen removal efficiency as a nitrogen getter and simultaneously adding to a solvent metal a material capable of suppressing formation of a carbide of Group IVa element, a material capable of diffusing the carbide or a material capable of improving the activity of carbon in the solvent metal, so that inclusions are not taken in the crystal. However, several ppm of boron is still included in the crystal, so that there are found an absorption of light due to boron in the infrared range and some strains or defects in the crystal.
As described above, natural diamond has a number of defects or large strains in the interior part of the crystal. Natural diamond of IIa type contains less impurities, but is not good as to the crystallinity such as defects, strains, etc. Thus, the natural diamond of IIa type has a problem that it tends to be cracked during working and when applying to technical fields needing a strength as diamond, for example, an anvil for producing an ultra-high pressure, compression cell for FT-IR, laser window material, etc., it is readily broken in some case. Further, it cannot be applied to a field needing high crystallinity, for example, monochomaters, semiconductor substrates, etc.
On the other hand, a synthetic diamond of IIa type is much more excellent in crystallinity than natural diamond, but is not sufficient in other properties, for example, such problems arising that the working yield is low, the mechanical strength is lower than that of diamond itself and the synthetic diamond of IIa type cannot be applied to a field needing high crystallinity, for example, monochomaters, semiconductor substrates, etc.
The synthetic diamond crystal of IIa type of the prior art contains several ppm of boron and thus meets with an absorption of light due to boron in the infrared range, thus resulting in a problem on the application to optical parts. In addition, there are some defects or strains in the crystal.
That is, in the synthesis of diamond by the temperature gradient method, diamond is used as a carbon source, but a commercially available diamond powder contains 10 to 1000 ppm of boron and natural diamond powder conatins several tens to several hundreds ppm of boron with a large dispersion. In the synthesis of diamond using such a carbon source, several ppm to ten and several ppm of boron is contained to give blue color. Accordingly, there occurs an absorption due to boron in the infrared range and ultraviolet to visible range, which is not preferable as an optical part. The concentration of boron is largely different depending on the growth sectors and the boron distribution is largely uneven in the interior part of the crystal. This is considered to be one reason for which the crystallinity is not good.
Measurement of the strain of diamond has hitherto been carried out by visual measurement using a polarizing microscope, but this method is not so precise in the quantitative respect.
There has been proposed a method of measuring the strain of diamond using Si (004) or Ge (004) commonly used as a first crystal in the double crystal X-ray diffraction method, but since the crystal interplanar spacing thereof used for the diffraction is not the same as diamond, the FWHM of a rocking curve is largely broadened (e.g. about 60 arcseconds) and this method does not lead to a precise and quantitative estimation of strain.
Under the situation, the present invention aims at providing a high purity synthetic diamond with less impurities, crystal defects, strains, etc., a process for the production of the same and a method of measuring the strain of the synthetic diamond.
As illustrated above, when boron is substantially completely removed, a colorless, transparent and defect- or strain-free crystal is obtained, but for this purpose, it is required to use very high purity raw materials of a carbon source and solvent and there arises a problem on the supply and cost of raw materials.